AC-powered LED light engine

ABSTRACT

AC LED light engines powered directly from the AC power line contain circuitry of resistors, capacitors, diodes and transistors which enables a single string LEDs connected to series to efficiently produce light with a relatively low level of flicker as perceived by the human eye. The LEDs are driven by a current which is alternately capacitively-limited and resistively-limited. Capacitively-limited pulses of current are interposed between resistively-limited pulses of current so that the resulting output current ripple is at frequencies of 240 Hz or above which the human eye cannot perceive. The combination of resistively-limited current and capacitively-limited current results in a current drain from the power line which is generally sinusoidal and can have a power factor in excess of 0.70.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the following US Provisionalapplications: Ser. Nos. 61/940,830 (filed Feb. 17, 2014), 62/034,540(filed Aug. 7, 2014), 62/041,192 (filed Aug. 25, 2014), and 62/056,591(filed Sep. 28, 2014), where all of these provisional applications areherein incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates in general to an alternating current (AC)-poweredlight emitting device (LED) arrangement, and more particularly to anAC-powered LED light engine that is capable of producing light outputwith relatively low flicker while providing the desired high lightemitting efficiency and power factor criteria.

Description of the Related Art

So-called AC LED diode products have become popular because onerelatively small, flat module can be joined directly to an AC power lineand will produce light suitable for many common illumination purposes.The very first such products consisted of LEDs mounted in antiparallelpairs, with as many as forty such pairs connected in series. Typically,a resistor was placed in series with such an arrangement to provide amore constant (and less voltage sensitive) light output. Such a priorart arrangement is shown in FIG. 1. Later, it was found to be moreeconomical to use a string of single diodes in place of theanti-parallel pairs, still with a limiting resistor (or later acurrent-controlled resistor). In this configuration, a bridge rectifierwas also required to be connected between the two ends of the string andthe AC power line. This type of prior art arrangement is shown in FIG.2.

One weakness of such products was that the light was only produced atthe peak of the AC power line, when the instantaneous power line voltageexceeded the sum of the forward voltages of the LEDs. This shortduration flash, repeated at 120 Hz, could cause stroboscopic effects andfor some vulnerable persons might even induce epileptic seizures.Technology was then developed in the form of integrated high voltageswitch arrays that were used to sense the AC line voltage, and to causeshorting switches to be closed across suitable numbers of the LEDs sothat the sum of the forward voltages of the unshorted LEDs at each pointin the power line cycle roughly matched the instantaneous power linevoltage. A schematic of a typical prior art product of this kind isshown in FIG. 3. These products have proven popular because they draw apower line current which is roughly sinusoidal in phase with the powerline voltage, resulting in power factor and total harmonic distortion(THD) numbers that are pleasing to the utility companies, while drivingthe LEDs with a net current that is roughly a half sine wave waveform.Therefore, instead of the intense light flash at the peak of each powerline half cycle, the light production (which is proportional to thetotal LED current) rises smoothly to a peak during a power line halfcycle and then declines again smoothly to zero before the next powerline half cycle.

However successful these products have been, they have not foundapplication in the most demanding applications—such as task lighting,workshop lighting and office lighting. This lack of success is due, atleast in part, to the fact that there is still a residual stroboscopiceffect in these switch-controlled arrangements. Although not noticeableby most people most of the time, this effect can produce headaches andeye strain if these light sources are used in these task, workshopand/or office light applications (among others). This fluctuation of thelight at 120 Hz is often referred to as “flicker” in the lightingindustry. The conventional definition of flicker is the fraction of theminimum point in the LED current waveform in terms of the maximum(I_(max)) and minimum (I_(min)) current levels in the waveform, thus:flicker=(I _(max) −I _(min))/(I _(max) +I _(min)).

This definition is useful for low frequency sinusoidal fluctuations in awaveform. However, defining flicker in this fashion may not relate wellto the perception of the human eye when extremely rapid fluctuations arepresent. The human eye cannot perceive any fluctuations at more than 120Hz frequency, and even at 120 Hz the perception is marginal. Therefore,when high frequency fluctuations are present, the conventionaldefinition of flicker does not comport to the perception of the humaneye. In particular, if tiny notches of 2 msecs or less are taken out ofan LED current waveform, the human eye cannot react fast enough tonotice them. This is why high frequency pulse width modulation of LEDcurrent is used to produce the perception of dimming of LED light.

It is clear from the foregoing that there is a need for an AC-driven LEDlight engine that can produce light with low flicker to pleaseconsumers, while still having all the attractive features of theexisting AC LED lights. A clue to the direction from which such acircuit might come was discussed in a paper entitled “A Driving Schemeto Reduce AC LED flicker” by Tan and Narendran, presented at the 2013SPIE meeting in San Diego. This paper came to the conclusion thatflicker could be minimized by the combination of capacitive andresistive drive to LEDs. However, the authors merely describe a parallelcombination of a pair of bidirectional LED strings, the activation ofone string of LEDs being resistively limited and the activation of theother, parallel string of LEDs being capacitively limited. The Tan etal. paper does not provide a practical engineering solution as to howthis could be done.

As described in the Tan et al. paper, the circuit presented will requireeither twice or four times the number of LEDs as the variousconfigurations of the present invention described hereinbelow. If the ACLEDs proposed by Tan et al. were replaced with LED strings enclosed bybridge rectifiers, fewer LEDs would be required, but the gap betweenlight outputs from each half cycle becomes a major disadvantage. U.S.Pat. No. 8,569,961 issued to Lee et al. on Oct. 29, 2013 presents acircuit based upon both resistive and capacitive coupling, with aprinciple using cross-coupled capacitors, but again requires twice asmany LEDs as the present invention.

SUMMARY OF THE INVENTION

While not intending to limit the scope of the claims or disclosure, inbrief summary, the present disclosure and claims are directed towards anAC-driven LED light engine which produces light output with relativelylow flicker, high efficiency and high power factor.

The present invention provides an AC LED drive circuit which uses aminimal number of LEDs and provides both high efficiency powerconversion and either continuous light output or minimal gaps betweenlight outputs from successive power line half cycles, resulting in lowflicker at 120 Hz, and a nearly sinusoidal power line current. Oneembodiment of the invention as described in detail below exhibits apower factor greater than 0.70.

Four different embodiments of the invention are described. They allshare the general concept that resistively-limited current passesthrough LEDs at the peaks of the power line voltage waveform, and inbetween these peaks, capacitively-limited current is passed through someof these same LEDs so that the peak light output from thecapacitively-limited current is comparable to the peak light output fromthe resistively-limited current. The net result is that the LED ripplecurrent is transformed from being a 120 Hz phenomenon (to which thehuman eye may react at times) into a 240 Hz (or higher) frequency, whichthe human eye cannot perceive. The AC LED current can be continuous insome of the embodiments and a power factor greater than 0.7 is achievedin at least one configuration of one embodiment of the presentinvention.

Disclosed and claimed in a first embodiment is an LED circuit in whichthe LEDs are arranged in one continuous string, subdivided for referenceinto four parts, designated as substrings A through D from the positiveend to the negative end. Substrings A and B are operated primarilyduring positive power line voltage half cycles (and thus referred to attimes as “the positive substrings”), while substrings C and D areoperated primarily during power line negative half cycles (“the negativesubstrings”). During the first part of a positive power line half cycle,segment B is driven into conduction (i.e., illuminated) by displacementcurrent through capacitors (the “capacitively-limited” current), andthen starting near the peak of the positive power line voltage halfcycle, both substrings A and B are illuminated via current that isresistively-limited from the power line (galvanic current). Before theend of the positive power line half cycle, substring C starts conductingdisplacement current from a capacitor which was precharged during theprevious half cycle. As before, close to the peak of the negativevoltage half cycle, substring D is illuminated by virtue of conductingresistively-limited current (galvanic current) directly from the powerline. In this manner, the current through substring B is firstcapacitively limited (displacement current) and then resistively limited(galvanic current). The same follows (during the opposite half cycle)for substring C. With repetition of this cycle, LED light continues tobe produced at a relatively uniform level, with only two brief near zerooutput points per power line cycle.

Disclosed and claimed in a second embodiment is an arrangement using thesame basic circuit as the first embodiment, but wherein solid stateswitches are incorporated so that substring B is shorted out whensubstring A has started conducting, and substring C is shorted out whensubstring D has started conducting. In this way, the resistively-limitedconduction period is extended to overlap with the subsequentcapacitively-limited operation. So-called “dead periods” are essentiallyeliminated in this embodiment and the current drawn from the power lineis more nearly in phase with the power line voltage, improving the powerfactor while the total LED current remains relatively constant. Thepower factor of an AC circuit is typically defined as a ratio of actualpower dissipated by the circuit to the product of its rms voltage andrms current. The power factor is generally used as a way to measure howefficiently the AC voltage line power is being used.

Disclosed and claimed in a third embodiment is a circuit in which twosubstrings of LEDs are connected to the power line by a capacitor attheir common point. This capacitor drives these two substrings like acharge pump, first pulling charge out of the one substring and thenpushing it into the other substring. A full bridge rectifier, connectedto the power line, has its output joined to the two ends of thesubstrings through resistors. Therefore, at the peak of the power linevoltage waveform, the resistively-limited current (galvanic current)passes through both substrings. In this manner, the LED current iscontinuous and the ripple current which is present is primarily at afrequency of 240 Hz or higher. The number and cost of the LEDs used inthis embodiment may be reduced when compared to the first twoembodiments, as a result of including a full bridge rectifier (i.e.,only a “positive” substring pair is required for use with the rectifiedcurrent).

Disclosed and claimed in a fourth embodiment is a circuit containingthree sequential strings of LEDs (denoted as α, β and γ) connected to afull bridge rectifier through resistors at either end, similar to thethird embodiment. In this particular fourth embodiment, capacitors aredirectly connected between the respective ends of the LED string and theAC power line. Diodes connect between the incoming power line to betweensubstrings α and β, as well as between substrings β and γ. LED currentconsists first of displacement current through one of the capacitors,and then of galvanic current through one of the resistors. Since thecapacitors are relatively small, the power factor can be greater than0.7 as required for US consumer Energy Star compliance.

Other features and advantages of the present invention will becomeapparent from the following description of the invention that refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic of a prior art light engine in which aresistor is used to limit the current flowing through a string ofbidirectional LEDs connected to the AC power line.

FIG. 2 is a simplified schematic of a prior art light engine in which aresistor is used to limit the current through a unidirectional string ofLEDs connected to a full bridge rectified AC power line.

FIG. 3 is a simplified schematic of a prior art light engine in whichthe AC power line is rectified and fed to a string of unidirectionalLEDs which have an array of switches connected so that the number ofLEDs connected to the power line can be continuously adjusted to matchthe power line voltage during the power line cycle.

FIG. 4 is a schematic of a first embodiment of the present invention.

FIG. 5 shows a comparison of the applied AC line voltage to the inputline current and the total LED current through all four LED substringsfor the first embodiment.

FIG. 6 shows a comparison of the applied AC line voltage with the summedcurrent through all the LED substrings and the individual currentthrough each of the four LED sub strings for the first embodiment.

FIG. 7 shows a comparison of the input line voltage with the summed LEDstring currents and the voltage across each of the bias capacitorsutilized in the first embodiment.

FIG. 8 shows a schematic of a second embodiment of the invention whichemploys solid state switches to ensure continuous light output andimproved power factor.

FIG. 9 shows the comparison of the applied AC line voltage with thesummed current through all the LED strings and the individual currentthrough each of the four substrings for the second embodiment as shownin FIG. 8.

FIG. 10 shows a comparison of the applied AC line voltage to the inputline current and the total LED current through all four substrings forthe second embodiment.

FIG. 11 shows a schematic of a third embodiment of the invention whichis cost reduced by using a full bridge rectifier to reduce the number ofLEDs used.

FIG. 12 shows the comparison of the applied AC line voltage with theindividual currents and the summed currents through both of the LEDsubstrings for the third embodiment of FIG. 11.

FIG. 13 shows a comparison of the applied AC line voltage to the inputline current and the total LED current through both LED substrings forthe third embodiment.

FIG. 14 shows a schematic of a fourth embodiment of the invention, whichis cost effective since it uses a full bridge rectifier and only usesrelatively small capacitors so that the power factor can be greater than0.7.

FIG. 15 shows the LED current through the individual LED substrings ofthe fourth embodiment, compared with the AC line voltage.

FIG. 16 shows the input and output current waveforms of the fourthembodiment and also displays the results for the efficiency and powerfactor.

DETAILED DESCRIPTION OF THE INVENTION

As described above, conventional arrangements for driving LEDs directlyfrom an AC power line may utilize switches to short out appropriatenumbers of LEDs so that the voltage being applied from the power line atevery instant in time roughly matches the requirements of thenon-shorted elements of the LED array. In accordance with the variousembodiments of the present invention as will be described in detailbelow, an alternative approach is proposed, where the LEDs are disposedin series as a single “string”, and the AC power is initially fed to afraction of the LEDs (defined as one or more “substrings”) usingdisplacement current though a capacitor (hereinafter referred to as“capacitively-limited current”). Then, once the power line voltage ishigh enough, a larger number of LEDs is driven directly from the powerline with a resistive type of current limiting element.

Although only a single capacitor is depicted in each of the embodimentsto drive one subsection of an LED string, it is to be understood thatthe principles of the invention could be extended to use multiplecapacitors driving corresponding multiple subsections of the LED stringin succession.

A first embodiment of an AC-driven LED light engine 10 of the inventionis depicted in FIG. 4. In this embodiment, a plurality of individualLEDs 12 are all connected in series in one continuous string. Forpurposes of operation, the string itself is subdivided into foursubstrings, shown as A, B, C and D in FIG. 4. Although it is convenientfor manufacturing purposes for each substring to have the same number ofindividual LEDs, there may be some circumstances where it isadvantageous to use different numbers of LEDs in each substring. In itsmost general form, LED light engine 10 can be configured to have anysuitable number of separate LEDs within each substring.

Indeed, in this and all other embodiments the number of LEDs used in asubstring will depend on the forward voltage of the specific LED devicesbeing used, as well as the intended power line voltage (for US consumerapplications, generally a 120V power line). By way of example only, theexemplary substrings of LEDs 12 as shown in FIG. 4 may each includetwenty-one individual LEDs (typical when utilizing commonplace whiteLEDs with a 120V power line).

As shown in FIG. 4, the connection between substrings B and C (denotedas midpoint M) is connected to a first termination 14 of an AC powerline 16, with the two opposing ends of the string (these opposing endsdesignated as X and Y in FIG. 4) connected to the second, opposingtermination 18 of power line 16. In particular, a first end X of thestring is connected to termination 18 through a series combination of adiode 20 and a resistor 22, and a second end Y of the string isconnected to termination 18 through a series combination of a diode 24and a resistor 26.

In accordance with this first embodiment of the present invention, afirst capacitor 28 is disposed across diode 24 and used to apply thechanging positive power line half-cycle voltage across a bias capacitor30 so that current is conducted through substring B of LEDs 12, and thenback to power line terminal 14. For the purposes of the presentinvention, the term “bias capacitor” is intended to describe the pair ofcapacitors utilized in each embodiment to control (i.e., limit) thecapacitively-limited current (displacement current) that flows throughthe LEDs at certain points in each power line half cycle. The biascapacitors are characterized by the property that they operate as apair, each associated with a half cycle of the voltage such that, ingeneral, one is discharging while the other is charging.

Returning to the description of the embodiment of FIG. 4, during thechanging negative power line voltage half cycles, a diode 32 is shownand functions to reset bias capacitor 30 during the negative power linevoltage half cycle so that bias capacitor is “ready” for the next halfcycle. For the negative power line voltage half cycles, a capacitor 34disposed across diode 20 has a complementary function to capacitor 28,while a bias capacitor 36 has a complementary function to bias capacitor30 (in this case, conducting current through substring C). Similarly, adiode 38 has a complementary function when compared to diode 32. Thecollection of capacitors 28, 30, 34 and 36 act together to constitute acapacitive means that shapes a distinctive capacitively controlled humpin the LED output current waveform.

At the peak of the positive power line half cycle, resistively-limitedcurrent flows directly from the power line, through diode 20 andresistor 22, along substrings A and B of LEDs 12, and then back to thepower line terminal 14. This is characterized as “galvanic” or“resistively-limited” current, as opposed to the so-called“displacement” or “capacitively-limited” current that flows through LEDs12 at other points in the power line cycle. During the peak of thenegative power line voltage half cycle, galvanic current flows frompower line terminal 14 through substrings C and D of LEDs 12, throughresistor 26 and then through diode 24 back to power line terminal 18.

It is to be understood that resistors 22 and 26 could be replaced, forexample, by a single resistor placed between midpoint M of LED substrings B and C and the connection of diodes 32 and 38, or by a singleresistor disposed between the connection of diodes 32 and 38 and the ACpower line 16. Resistors 22 and 26 could also be distributed throughoutthe LED substrings and those skilled in the art will see numerous otherways to implement this resistive means for providing a functionality tocontrol the resistive-limited current.

While the above explanation serves to describe the basic operation ofthe present invention, there are certain advantages and features of theoperation which lead to a relatively uniform total LED current with onlyvery narrow gaps of no conduction. These advantages are explained inassociation with the signal plots shown in FIG. 5. In particular, plot(a) in FIG. 5 shows the waveform of the typical AC input line voltageand plot (b) depicts the associated input line current. The summedcurrent through all four substrings A-D of LEDs 12 (i.e., the total LEDcurrent), is shown in plot (c). Since LED light output (illumination) isproportional to the LED current at low current densities, this summedcurrent is accurately representative of the illumination if all foursubstrings are identical. It can be seen that the summed LED current isconfined to a relatively narrow range R between 200 mA and 300 mA,except for a sharp, brief 1.5 msec period P every half cycle. The humaneye cannot perceive such rapid fluctuations and simply perceives thislight output as being smooth and continuous.

A unique feature of this exemplary embodiment of the present inventionis that the reason the gap between half cycles is so short is becausethe LED current corresponding to the “next” power line voltage halfcycle commences about halfway through the descent portion of the“present” voltage half cycle. This overlap in powering differentsubstrings, in accordance with the present invention, is the result ofbias capacitor 30 being discharged during the positive voltage halfcycle (where bias capacitor 30 will be recharged during the subsequentnegative voltage half cycle). Similarly, bias capacitor 36 is dischargedduring each negative voltage half cycle and then recharged during thesubsequent positive voltage half cycle. Thus, once a positive voltagehalf cycle starts declining, bias capacitor 36 is fully charged andstarts delivering current which will flow through substring C during thenegative voltage half cycle. Once a negative voltage half cycle startsdeclining, capacitor 30 (which is now fully charged) starts deliveringcurrent which will flow through substring B during the positive voltagehalf cycle.

In FIG. 6 the summed (i.e., total) LED current is plotted out forcomparison with the input line power voltage. The plots of the LEDcurrent passing through each individual substring A-D are also containedin FIG. 6. As shown, as the positive line power voltage half cyclestarts to rise, the current through substring B of LEDs 12 risesabruptly and then steadily declines (with reference, for example to thecurrent through substring B at 172 msec). There is an inflection in thedeclining curve, shown at point I, where the line voltage peak issufficiently high to turn on substring A of LEDs 12. Although thecurrent through substring B of LEDs 12 continues to decline, the risingcurrent through substring A means that the summed LED current remainsbetween 200 mA and 300 mA. By virtue of the symmetry of thisconfiguration, the same current flows through substrings C and D occurduring a negative line power voltage half cycle time period.

To explain the sharp rise in LED current once the power line voltagestarts to decline from its peak, it is helpful to look at FIG. 7. Here,the voltage waveforms across bias capacitors 36 and 30 are plotted outfor comparison with the AC line voltage, with a plot of the summed LEDcurrents also shown in FIG. 7. During the positive half cycle, biascapacitor 36 has become charged to roughly 180V, equal to the peak ofthe line voltage. Each of the LED substrings requires about 73V to beactivated. Therefore, when the power line voltage peak starts todecline, the declining voltage is conveyed to a first terminal of biascapacitor 36 by capacitor 34, causing the anode of diode 38 to be pulledto a more negative voltage, turning on substring C of LEDs 12. Sincebias capacitor 36 is fully charged, any additional decline in the linevoltage peak results in the application of more voltage across substringC. This results in the LED current rising very sharply, making theinterval between conduction periods only about 1.5 msecs. The sharp riseand fall times of these currents, therefore, is controlled by thepresence of capacitors 28 and 34. Prior art LED light engines, such asthose mentioned above, used simple resistor and capacitor circuits andas a result were only able to achieve a 3 msec gap between currentpulses, since these circuits did not charge one bias capacitor while theother bias capacitor was being discharged.

Exemplary Embodiment

In order to demonstrate the first embodiment of the invention thefollowing component values were used, although clearly many variationsof these values are possible within the spirit of the invention. The LEDsubstrings contained 21 white LEDs. Capacitor 36 and capacitor 30 were10 μF each. Capacitors 34 and 28 were 7.5 μF. The rectifying diodes wereMUR140 types. Resistors 22 and 26 were 56Ω. The power factor of thisembodiment was 0.45.

A second embodiment of the invention is depicted in FIG. 8 as an LEDlight engine 40. Those components of LED engine 40 that provide the samefunctionality as LED engine 10 of FIG. 4 are shown by the same referencenumeral. What is different in this particular embodiment is theinclusion of a pair of current-triggered switches that are placed acrosssubstrings B and C of LEDs 12 and are used to “short out” thesesubstrings during the resistive-limited current portion of each halfcycle. A first current-triggered switch 42 is disposed across substringB and is activated to bypass substring B by when the current throughresistor 22 reaches its peak (positive) value. Similarly, a secondcurrent-triggered switch 44 is disposed across substring C and isactivated to bypass substring C when the current through resistor 26reaches its peak (negative) value. In this embodiment, therefore,substrings A and D are primarily used to supply LED current in the peakregions of the AC power line input, with substrings B and C used tosupply LED current in the transition times between the peaks in the ACvoltage cycle.

Referring now to FIG. 8 in detail, first current-triggered switch 42comprises an NPN transistor 46 in series with a current-limitingresistor 48 across substring B. NPN transistor 46 is turned on when thecurrent through resistor 22 produces a voltage sufficient to extract abase current from a series combination of an associated PNP transistor50 and resistor 52. When the current through resistor 22 is largeenough, PNP transistor 50 is activated and sends a drain current througha resistor 54, which will then begin to charge a capacitor 56. After anRC time delay determined by the combination of resistor 54 and capacitor56, capacitor 56 becomes sufficiently charged to drive a base currentinto NPN transistor 46 through a resistor 58, turning on NPN transistor46 and shorting out LED substring B. As a result, NPN transistor 46 thenpulls drain current through resistor 48, thereby increasing the currentthrough resistor 22, which started the whole process.

The end result is that transistors 46 and 50 become latched “on” in aregenerative process until the power line voltage becomes less than theforward voltage across substring A of LEDs 12 (i.e., towards the end ofthe positive half-cycle of the AC power line voltage). Prior to thisthreshold-crossing in the cyclic AC power line voltage, substring C ofLEDs 12 has already commenced operation (in response to displacementcurrent through bias capacitor 36, described above), thus ensuring anoverlap between the light outputs from the two substrings B and C. Thatis, by virtue of using both a galvanic (resistively-created) current anda displacement (capacitively-created) current, there is no perceptiblegap in the application of energy to the LEDs.

An exactly complementary process takes place within secondcurrent-triggered switch 44 during the negative half cycles along the ACpower line, with a pair of transistors 60 and 62 latching “on” to shortout substring C of LEDs 12 later in the cycle. In particular, secondcurrent-triggered switch 44 includes a series combination of PNPtransistor 62 and a current-limiting resistor 64 disposed acrosssubstring C. PNP transistor 62 will turn on when the current throughresistor 26 produces a voltage sufficient to extract a base currentthrough a combination of NPN transistor 60 and a resistor 66. That is,when the current through resistor 26 is large enough to activate NPNtransistor 60, the current flowing through transistor 60 will passthrough a resistor 68 and begin to charge a capacitor 70. The RC delayassociated with the combination of resistor 68 and capacitor 70 thusdetermines the time when capacitor 70 becomes sufficient charged todrive a base current into PNP transistor 62 through a resistor 72,turning on PNP transistor 62 and bypassing substring C. Thus, PNPtransistor 62 pulls current through resistor 64 and thereafter increasesthe current through resistor 26. As with first switch 42, this action ofsecond switch 44 results in transistors 60 and 62 becoming latched “on”in a regenerative process until the magnitude of the power line voltagebecomes less than the forward voltage across substring D of LEDs 12.

FIG. 9 shows the total LED current for LED light engine 40 of FIG. 8,provided for the sake of comparison with the current through each of thefour substrings A-D. It can be seen that the current pulses associatedwith separate pairs of substrings overlap, in accordance with theinventive use of both resistive (galvanic) and capacitive (displacement)current. Therefore, as shown in the graph of the total LED current, theaverage total LED current is about 300 mA, and the lowest total LEDcurrent is 100 mA (which is only approached very briefly for 1.5 msecsat a time). In FIG. 10, the input current and total LED current for LEDlight engine 40 are shown in comparison with the input AC line voltage.It can be seen that for the configuration of LED light engine 40 of FIG.8, the input current is more nearly in phase with the input line voltagethan was the case with the first embodiment (LED engine 10, FIG. 4), asreviewed in comparison with the diagram of FIG. 5. Thus, the powerfactor is improved for this configuration with respect to that of FIG.4. Additionally, it is clear that the input line current waveform asshown in FIG. 10 is closer to a sine wave (although still somewhatdistorted) than the input line current waveform associated with LEDengine 10 as shown in FIG. 5.

Exemplary Embodiment

In order to demonstrate the second embodiment of the invention thefollowing component values were used, although clearly many variationsof these values are possible within the spirit of the invention. Thefour LED substrings each comprised twenty-one white LEDs. Biascapacitors 30 and 36 were each 10 μF. Capacitors 28 and 34 were each 7.7μF. Capacitors 56 and 70 were 1.0 μF. Diodes 20, 24, 32 and 38 were typeMUR140. NPN transistors 46 and 60 were type BUW40. PNP transistors 50and 62 were type RCA30C. Resistors 52, 58, 66 and 72 were 470Ω.Resistors 22 and 26 were 75Ω. Resistors 54 and 68 were 4.7KΩ. Resistors48 and 64 were 300Ω. The power factor of this embodiment was 0.62.

A third embodiment of the invention is shown as LED light engine 80 inFIG. 11. In this configuration, light engine 80 utilizes only a pair ofLED substrings, denoted as substrings I and II in FIG. 11. At theirmidpoint M, a capacitor 82 is connected directly to a first terminal 84of a power line connector 86. A pair of bias capacitors 88 and 90 isused to conduct displacement current directly back to an opposingterminal 92 of power line connector 86. These capacitors, together withcapacitor 82, constitute the capacitive means responsible for producingthe distinctive hump associated with displacement current in the outputcurrent waveform, as particularly shown in FIG. 12.

A set of diodes 94, 96, 98 and 100 comprises a full bridge rectifierarrangement that provides rectified power to the opposing ends ofsubstrings I and II through resistors 102 and 104, respectively. Inparticular, resistors 102 and 104 constitute the resistive means fordriving the LEDs. It is to be remembered, as mentioned before, thatvarious arrangements may be used to provide this resistive means, whichvarious configurations are considered to be well known to those skilledin the art. An additional capacitor 106 is disposed between terminals Xand Y, and is used to smooth the output voltage from the bridgerectifier.

FIG. 12 is associated with LED light engine 80 and shows the currentthrough each substring I and II individually, as well as the summed LEDcurrent. All of these are shown in comparison with the AC power linevoltage. It can be seen that the currents from substrings I and IIoverlap each other, shown in the shaded regions S of FIG. 12, so thatthe summed string current is continuous throughout the line voltagecycle. As a result of this overlap, the ripple current present on thesummed string current is primarily fluctuating at a frequency of 240 Hzand above, which is imperceptible to the human eye. Since thisembodiment requires only half the number of LEDs as the other twoembodiments discussed thus, it is especially cost effective (thereduction in the number of LEDs attributed to the use of the rectifier).In FIG. 13, the input power line current is shown by comparison with thepower line voltage and the summed output LED string currents. It can beseen that the input current of LED light engine 80, as shown in FIG. 13,is of a generally sinusoidal nature.

Exemplary Embodiment

In order to demonstrate the third embodiment of the invention thefollowing component values were used, although clearly many variationsof these values are possible within the spirit of the invention. The twoLED substrings (I and II) each comprised twenty-one white LEDs,capacitors 88 and 90 were each 4.7 μF, capacitor 82 was 5.6 μF,resistors 102 and 104 were 36Ω, capacitor 106 was 3.3 μF and diodes 94,96, 98 and 100 were type MUR160. The power factor of this embodiment was0.45 (the reduction in power factor when compared to the secondembodiment primarily the result of the phase mismatch between the inputline voltage and the input line current).

A fourth embodiment of the invention, denoted as LED light engine 120,is shown in FIG. 14. This embodiment provides a relatively low-costconfiguration, requiring only between fifty and sixty total LEDs, withefficiency on the order of 90% and a power factor greater than 0.7 (asis required for example, for Energy Star qualification in the UnitedStates). Although at first glance LED light engine 120 may appear to bedifferent from the embodiments described above, a pair of biascapacitors 122 and 124 performs the same function (i.e., energy storage)as bias capacitors 36 and 30 of light engines 10 and 40, and biascapacitors 88 and 90 in light engine 80. Similar to the otherembodiments, these capacitors comprise the capacitive means that producethe distinctive capacitively-limited current hump in the output currentwaveform. Resistors 126 and 128 perform the same current-limitingfunction as, for example, resistors 22 and 26 in LED light engine 10 ofFIG. 4, and constitute the resistive means as described hereinabove withthe other embodiments of the present invention.

In accordance with this embodiment of the present invention, a pair ofdiodes 130 and 132 is used to initially activate only a fraction of theLEDs 12 (in this embodiment, LEDs 12 are depicted as a set of threesubstrings, denoted α, β and γ). A node N between bias capacitors 122and 124 is used for purposes of explanation as a reference or commonnode, relative to which voltages in the circuit are judged to be eitherpositive or negative. Also shown in this embodiment is a bridgerectifier formed of a set of diodes 134, 136, 138 and 140. As input ACvoltage rises at the connection of diodes 134 and 136, bias capacitor122 becomes positively charged (through diode 134 and resistor 126). Itis to be noted that bias capacitor 124 was earlier charged negativelyduring a previous half cycle. Therefore, as soon as a slightly positivevoltage appears on the cathode of diode 130, current will begin to flowthrough LED substrings β and γ, thus discharging bias capacitor 124.Eventually, as bias capacitor 124 becomes discharged, the incoming linevoltage has risen to a sufficient extent that current can now passthrough diode 130 and LED substrings β and γ, and thereafter throughresistor 128 back into the power line through diode 136.

On the subsequent line voltage half cycle, in which the voltage on thecenter point of diodes 134 and 136 is falling, as soon as the voltage onthe anode of diode 132 starts to go negative, the voltage which isalready stored on bias capacitor 122 will drive displacement currentthrough LED substrings α and β. As before, at the point where biascapacitor 122 is almost completely discharged, galvanic (i.e.,resistively-limited) current continues to flow through resistor 126 toenergize LED substrings α and β.

FIG. 15 shows the current through each of the LED substrings during apower line cycle. Each pulse of LED current shows the two distinctivehalves: a first half in which the current is capacitively limited(displacement current) and then a second half in which the current isresistively limited (galvanic current). The summed LED current waveformshown in FIG. 15 exhibits the same, distinctive two-hump pattern as ispresent in the total LED current waveform of FIG. 5, confirming that thesame basic mechanism is at work. FIG. 16 shows the input line current bycomparison with the input line voltage. Since the input line current nowchanges polarity closer to the line voltage zero crossing than in theprevious embodiments, the power factor is relatively higher,approximately 0.71, compared to the earlier embodiments.

Exemplar Embodiment

In order to demonstrate the fourth embodiment, a prototype used thefollowing components. The LEDs were a total of fifty-two white LEDs,with substrings α and γ each having thirteen individual LEDs. All therectifier diodes were type MUR160, although almost any diode withsufficient voltage and current capability could have served the purpose.Bias capacitors 122 and 124 were 2.7 μF and resistors 126 and 128 were100Ω each. With 120V applied, LED light engine 120 had an input power of17.5 W, with a power factor of 0.71. The electrical efficiency was 90%.This circuit is particularly attractive for general purpose use becauseof the low cost of the components, as well as exhibiting a power factorgreater than 0.7, the latter making it acceptable for Energy Starconsumer purposes.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It isanticipated that numerous other topologies can be created using the samebasic principles. The rectifying diodes used could be Schottky diodes orbipolar diodes. Any of the rectifying diodes could be made out of LEDstrings, as well as bipolar or Schottky diodes. The switches describedcould be made using MOSFETs, SCRs, IGBTs or any of the numerous solidstate switches known to those skilled in the art. The detailed switchtriggering mechanism used could be embodied in numerous ways. Theembodiments shown were optimized for more constant LED current, howeverby trivial modification of the circuit parameters they could also bemodified to give better power factor and THD at the expense of theuniformity of the LED current. Where LEDs are described as beingconnected anode to cathode, it is well known to those skilled in the artthat in each case one LED could be replaced by a parallel connection oftwo or more LEDs having the same orientation, so that if one LED failsthen the other can still conduct current and provide light in order toenhance the reliability of the assembly. The resistors described couldbe replaced by constant current diodes or other constant currentcircuits, combined if necessary with bypass diodes. Alternatively, theresistors could be distributed in different parts of the circuit; forexample, in combination with the LEDs or the rectifying diodes.

It is preferred therefore, that the present invention be limited not bythe specific disclosure herein, but only by the appended claims.

What is claimed is:
 1. A driverless light emitting diode (LED) lightengine for providing illumination from an AC voltage source, thedriverless LED light engine comprising: a plurality of individual LEDsconnected in series as a single string of LEDs, defined as comprising aplurality of LED substrings; resistance means disposed along a signalpath including the AC voltage source and the single string of LEDs; andat least two separate bias capacitors connected between the AC voltageand one or more LED substrings, the at least two bias capacitorssupplying capacitively-limited current so that at some points in timeduring an AC voltage cycle, at least one LED substring receivescapacitively-limited current, and at other points in time during said ACvoltage cycle said at least one LED substring receivesresistively-limited current.
 2. A driverless LED light engine havingfirst and second input terminals connected to an AC power source, theLED light engine comprising: a plurality of LEDs connected in series asan LED string having a first end termination and a second, opposing endtermination, the LED string defined as comprising a plurality of LEDsubstrings with a midpoint of the LED string connected to the firstinput terminal; a first rectifier diode coupled between the second inputterminal of the AC power source and the first end termination of the LEDstring so as to permit current flow during positive voltage half cycles;a second rectifier diode coupled between the second input terminal ofthe AC power source and the second end termination of the LED string soas to permit current flow during negative voltage half cycles; a firstcapacitor connected from a cathode of the first rectifier diode to anintermediate point on the LED string between the midpoint and the secondtermination so as to be in parallel with a first plurality of LEDsubstrings; and a second capacitor connected from an anode of the secondrectifier diode anode to an intermediate point on the LED string betweenthe midpoint and the first termination so as to be in parallel with asecond plurality of LED substrings.
 3. The driverless LED light engineas defined in claim 2 wherein the light engine further comprisesresistive means associated with the LED string to limit galvanic currentfrom the AC power source to the LED string.
 4. The driverless LED lightengine as defined in claim 2 in which a capacitor is placed in parallelwith the first rectifier diode and a capacitor is placed in parallelwith the second rectifier diode.
 5. The driverless LED light engine asdefined in claim 2 in which at least a third rectifier diode isconnected in antiparallel with an LED substring disposed between aconnection of said first capacitor and the midpoint of the LED string,and at least a fourth rectifier diode is connected in antiparallel witha separate LED substring disposed between a connection of said secondcapacitor and the midpoint of the LED string.
 6. A driverless LED lightengine having first and second AC power line terminals, the light enginecomprising: a plurality of single LEDs connected in series as a stringof LEDs, the string of LEDs defined as having a first end terminationand a second, opposing end termination, with a midpoint of the string ofLEDs connected to the first AC power line terminal; a first rectifierdiode connected between the first end termination of the string of LEDsand the second AC power line terminal; a second rectifier diodeconnected between the second end termination of the string of LEDs andthe second AC power line terminal, wherein a first half of the string ofLEDs conducts galvanic current during positive power line voltage halfcycles from an AC power source and a second half of the string of LEDsconducts galvanic current during negative power line voltage half cyclesfrom the AC power source; a first bias capacitor connected across aportion of the string of LEDs in such a manner as to cause a substringof LEDs in the first half of the string of LEDs to be illuminated beforethe beginning of the power line voltage half cycle corresponding to thegalvanic forward conduction of the first half of the LED string; and asecond bias capacitor connected across a portion of the string of LEDsin such a manner as to cause a substring of LEDs in the second half ofthe string of LEDs to be illuminated before the beginning of the powerline voltage half cycle corresponding to the galvanic forward conductionof the second half of the LED string.
 7. The driverless LED light engineas defined in claim 6 wherein the light engine further comprisesresistive means disposed in series with the string of LEDs so as tolimit the galvanic current from the power line through the plurality ofsingle LEDs.
 8. The driverless LED light engine as defined in claim 6wherein the first bias capacitor is connected from a terminal of thefirst rectifier diode to an intermediate point between the midpoint andthe second end termination of the string of LEDs; and the second biascapacitor is connected from a terminal of the second rectifier diode toan intermediate point between the midpoint and the first end terminationof the string of LEDs.
 9. The driverless LED light engine of claim 6wherein the light engine further comprises a first capacitor disposedacross the first rectifier diode; and a second capacitor disposed acrossthe second rectifier diode.
 10. The driverless LED light engine of claim6, wherein the light engine further comprises: antiparallel rectifierdiodes connected across selected substrings of LEDs forming the stringof LEDs.
 11. The driverless LED light engine of claim 6, wherein thestring of LEDs comprises a plurality of four LED substrings, with afirst LED substring and a second LED substring connected in seriesbetween the first end termination and the midpoint of the string ofLEDs, and a third LED substring and a fourth LED substring connected inseries between the midpoint and the second end termination of the stringof LEDs.
 12. The driverless LED light engine of claim 11 wherein thesecond and third LED substrings conduct both capacitively-limited andresistively-limited current, and the first and fourth LED substringsconduct resistively-limited current.
 13. The driverless LED light engineof claim 12, wherein the light engine further comprises a first bypassswitch disposed to short out the second LED substring when the first LEDsubstring is conducting to prevent resistively-limited current frompassing through the second LED substring; and a second bypass switchdisposed to short out the third LED substring when the fourth LEDsubstring is conducting to prevent resistively-limited current formpassing through the third LED substring.